Yahoo (NSDQ: YHOO) is making another acquisition in the online advertising space, after its buyout of RightMedia: it is now buying BlueLithium, the online ad network, for about $300 million in cash. According to comScore, BlueLithium is the fifth largest ad network in the US and second largest in the UK, and has about 145 million unique visitors each month.

Among BlueLithium’s product capabilities, which will help Yahoo: audience targeting based on consumer interests; remarketing ads to consumers across the Web who have interacted with an ad or web page; custom segmentation; spot buying capabilities to extend reach and frequency against a marketer’s target audience.

BL’s CEO Gurbaksh Chahal will remain with BlueLithium for an interim period through the integration..the deal is expected to go through in Q4 this year. The company founded in 2004 and based in San Jose, CA, had investment from Walden Venture Capital and 3i. More in release.

WSJ: The company, which has 120 employees and declines to disclose its revenue, had previously planned to hold an IPO early next year.

Yahoo’s own blog: The universe for Yahoo now includes inventory on Yahoo’s owned and operated properties, its affiliate network (partnerships with eBay, Comcast, and the consortium of nearly 400 newspapers), the Yahoo Publisher Network, and the Right Media Exchange.